Solved it on my own, but posting results for others:
Add this into your ASP code section at the beginning of your page:
<%
MinDate = DateAdd("d", 5, DATE())
MaxDate = DateAdd("d", 45, DATE())
%>
Then change your validation behavior to this:
WAValidateDT(document.forms[0].deliverydate,'- The date must be between <% Response.Write(MinDate) %> and <% Response.Write(MaxDate) %>',true,/\b(1[0-2]|0?[1-9])\/([12]\d|3[0-1]|0?[1-9])\/\d{4}\b/,'','<% Response.Write(MinDate) %>','<% Response.Write(MaxDate) %>',false,/.*/,'','','',document.forms[0].deliverydate,0,true);WAAlertErrors('The following errors were detected','',true,false,false);return document.MM_returnValue"
